One of the smallest electronic devices in widespread use is a Radio-Frequency Identification (RFID) tag. Rather than run on battery power, power is applied from an external source, such as an RFID reader or scanner. A small antenna on the RFID tag or device receives radio waves from the RFID reader. The received RF waves are rectified by an Analog Front End (AFE) to create a power-supply voltage. The AFE also extracts a coded signal that carries instructions or commands from the RFID reader.
FIG. 1 shows a RFID device that generates power by rectifying received RF waves. Matching network 104 is connected to a small antenna on the RFID device. The received RF signal is applied to rectifier 108 inside RF analog front-end 102. Rectifier 108 generates a power-supply voltage and current that powers RF analog front-end 102, digital base-band 106, and RFID processor 110, which may be integrated together as a single silicon chip, or may be separate chips. RF analog front-end 102 also extracts a signal that is carried by the RF waves, which may be encoded using a simple NRZ coding or using more complex coding schemes.
FIG. 2A shows a prior-art diode rectifier bridge circuit. Alternating-current source 114 produces an alternating wave such as sine waves on its AC+, AC− terminals. A full-wave rectifier bridge circuit is formed by diodes 116, 118, 120, 124 which are connected across terminals AC+, AC−. Load 112 is connected across intermediate nodes VDD, VSS in the diode bridge. Load 112 may be a resistor or may be a dummy load that represents a large electrical system load that is powered by VDD.
During a first phase of the AC wave, when AC+ is positive and AC− is negative, current flows from AC+, through diode 118 to VDD, then through load 112 to VSS, and finally through diode 124 to AC−. During a second phase of the AC wave, when AC+ is negative and AC− is positive, current flows from AC−, through diode 120 to VDD, then through load 112 to VSS, and finally through diode 116 to AC+.
Traditionally, alternating-current source 114 is an A.C. current such as from a 120-volt A.C. electrical wall socket. However, small electrical systems such as an RFID device is powered by a received RF signal. Since the antenna is small, the received RF signal is small and produces a relatively small voltage. Perhaps only a few volts are produced.
Diodes 116, 118, 120, 124 each produce a voltage drop of about 0.5 to 0.8 volts due to the p-n junction inside the diode. Thus VDD is one diode voltage drop below AC+, and VSS is one diode voltage drop above AC− at the peak of the first phase.
FIG. 2B shows diode voltage drops when rectifying with the diode rectifier of FIG. 2A. Power-supply voltage VDD (solid line) is a rectified wave that is generated by the full wave of AC+ (dashed line) and AC− (not shown). Since the AC current passes through two diodes, VDD is reduced by a voltage drop of 2 diode junction thresholds, or 2*VT.
When the diode rectifier bridge of FIG. 2A is used as rectifier 108 in an RFID system such as shown in FIG. 1, the RF signal from matching network 104 must be at least 2*VT greater in peak voltage than the desired VDD needed to power circuits such as RFID processor 110. A further voltage loss may occur when a power capacitor is added to VDD to smooth or average the rectified wave to produce a more stable VDD. A larger antenna may be needed to capture enough RF energy to produce a sufficiently large A.C. voltage. Of course, larger antennas are undesirable.
The diode rectifier bridge may be constructed from discrete diodes rather than integrated onto a same silicon substrate as RF analog front-end 102. This is also undesirable since the discrete diodes increase the cost and size of the RFID device.
What is desired is a full-wave rectifier that does not use discrete diodes. A rectifier that uses common transistors such as complementary metal-oxide-semiconductor (CMOS) transistors is desirable so that the rectifier can be integrated onto a larger system on a chip or AFE. A full-wave bridge rectifier that uses transistors rather than diodes is desired to avoid voltage drops due to p-n junctions in the diodes. A rectifier for an on-chip AC-to-DC converter is desired that can operate with a very small A.C. voltage is desired.